Spin-polarized tunnel injection and extraction of charge carriers can give rise to magneto-resistance in organic spin valves. To describe this magneto-resistance, the tunneling process is modeled as a transfer of electrons through a thin insulating layer between a ferromagnetic contact and an organic semiconductor. Transition rates between extended states in the metal and model molecular orbitals localized at the semiconductor/insulator interface are calculated based on a transfer Hamiltonian. The transition rates are then used in a rate equation model to calculate the injected current for the two spin types and the associated magneto-resistance of organic spin valves. Consistent with experimental data, it is found that the magneto-resistance can be of either sign and its magnitude strongly decreases with the applied bias.
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The authors thank Alex Morozoff, Divya Venkataraman, and Katie Moenkhaus for their assistance. This work was supported in part by the NSF (ECCS – 0724886) and DARPA (FA23861114058). Access to the facilities of the Minnesota Supercomputing Institute for Digital Simulation and Advanced Computation is gratefully acknowledged. Work at Los Alamos National Laboratory was supported by the DOE Office of Basic Energy Sciences Work Proposal No. 08SPCE973.